Shock absorbers of this kind according to prior art are usually called twin-tube dampers. Twin-tube dampers of this kind are used as passive shock absorbers, both as non-adjustable shock absorbers and as adjustable shock absorbers.
In the case of a non-adjustable twin-tube damper, the properties of the damper or of the damping are clearly defined in such a way that the flow resistance of the working medium is set by the valve arrangements used in the damper.
Adjustable twin-tube dampers are used when the work to be achieved by the shock absorber is to be adapted to corresponding vehicle states of a vehicle, in which an adjustable twin-tube damper of this kind is fitted, or to the sprung masses of different sizes respectively. In the case of passive, hydraulically adjustable twin-tube dampers of this kind, this is achieved in that the flow resistance can be modified by means of a variable valve hole in the valve arrangement arranged on the piston between the upper and the lower chamber of the inner pipe of the damper.
A further feature of passive dampers that are operated in a non-adjustable manner is that the amount of oil exchanged during a defined working stroke is always the same. The flow resistance is dependent only on the speed of the piston inside the inner pipe.
In the case of adjustably operated passive dampers, when the stroke is likewise defined the same, the flow resistance is influenced by the change in the valve cross section of the valve arrangement in the piston, in addition to the speed of the piston. The amount of oil exchanged is likewise the same for a defined stroke.
The mode of operation of a twin-tube damper of this kind according to the prior art is briefly described below.
When the damper is retracted, the piston moves downwards in the inner pipe so that the lower chamber of the inner pipe decreases in volume, whilst the upper chamber of the inner pipe increases in volume. During this retraction of the damper, a certain amount of oil, serving here as the working medium, is channelled from the lower chamber into the upper chamber via the valve arrangement of the piston, a defined flow resistance thus being generated across the valve arrangement of the piston by means of the speed thereof. The amount of oil in the upper chamber of the inner pipe that is additionally displaced in the process by the plunging of the piston rod is channelled off into the housing serving as a tank for the working medium or oil respectively via the valve arrangement arranged at upper end of the inner pipe.
If the damper is now withdrawn, the piston moves upwards inside the inner pipe, reducing the volume of the upper chamber and increasing the volume of the lower chamber, the amount of oil or working medium respectively to be exchanged being channelled from the upper chamber into the inside of the housing serving as a tank for the working medium or into the oil sump there respectively via the valve arrangement at the upper end of the inner pipe. The difference in the amount oil or working medium respectively which is required due to the withdrawal of the piston rod is drawn from the inside of the housing serving as a tank for the working medium or from the oil sump there respectively via the valve arrangement arranged at the bottom of the inner pipe.
As just explained, the working medium or oil respectively moves in a circuit inside the damper.
Since a twin-tube damper of this kind must be appropriately preassembled according to its use (passive adjustable or passive non-adjustable), it is not possible to use a damper provided in one case for another intended use.